Sedimentary Environments

1
Chapter 5

• Sedimentary Environments

2
Paleogeography

• Reconstruction of ancient environments from the
  stratigraphic record
• Distribution of land and sea
• Identification of localized environmental
  features
• Framework for interpretation of past life
• Employ actualism

3
Nonmarine Environments

• Soil
• Loose sediment containing organic matter and
  accumulated in contact with atmosphere
• Topsoil
• Upper zone of many soils
• Sand and clay mixed with humus
• Humus
• Organic matter that gives topsoil its dark color
• Derived from decay of plant debris by bacteria

4
Nonmarine Environments

• Soils
• Type of soil depends on climatic conditions
• Caliche
• Calcium carbonate produced by evaporation of
  groundwater
• Laterite
• Iron oxide rich soil produced in moist tropical
  regions

5
Nonmarine Environments

• Burrows
• Plant
• Animal
• Aid in identifying ancient soils
• Devils corkscrews

6
Nonmarine Environments

• Lakes
• Lower elevation, more likely preservation
• Indicates abundant precipitation
• Sediments
• Coarsest around lake margin
• Finest at center
• Often layered
• Freshwater fossils

7
Nonmarine Environments

• Glaciers
• Indicate cold climates
• Scratches produced by glacial motion
• Record direction of motion

8
Nonmarine Environments

• Till
• Unsorted, heterogeneous material
• Tillite
• Lithified till
• Moraine
• Ridges of till plowed up at the farthest edge of
  the glacier

9
Nonmarine Environments

• Meltwater
• Transports sediments from glacier
• Forms streams and lakes
• Outwash
• Well-stratified layers of sediment
• Varve
• Annual layers of alternating coarse and fine
  sediments

10
Nonmarine Environments

• Varves
• Annual record
• Count!
• Dropstones
• Scattered coarse sediments found in sediment
  matrix
• Ice-rafted debris

11
Lacustrine Environments (Lakes)

• May be large or small
• May be shallow or deep
• Filled with terrigenous, carbonate, or evaporitic
  sediments
• Sediments are typically fine grained but may be
  coarse near the edges
• Fine sediment and organic matter settling in some
  lakes produced laminated oil shales
• Playa lakes are shallow, temporary lakes that
  form in arid regions They periodically dry up as
  a result of evaporation

12
Nonmarine Environments

• Desert soils
• Little organic matter
• Evaporite minerals
• Interior drainage
• Precipitation does not leave the basin
• Playa lake
• Temporary lakes
• Associated with evaporites

13
Nonmarine Environments

• Dunes
• Piles of sand
• lt 1 of deserts
• Moves with prevailing wind direction
• Associated with deserts and beaches

14
Eolian Environments

• Wind is the agent of sediment transport and
  deposition
• Dominated by sand and silt
• Common in many desert regions

15
Nonmarine Environments

• Dune migration
• Moves downwind
• Sands move up and over top accumulate on
  downwind side

• Trough cross- stratification
• Direction changes with prevailing winds
• Beds accumulate on curved surface cut through
  older beds

16
Nonmarine Environments

• Alluvial fans
• Low, cone-shaped structures
• Develop where mountain slope meets valley floor
• Change in slope reduces stream velocity
• Sediments settle out
• Braided streams form

17
Alluvial Fans

1. Fan-shaped deposits at base of mountains.
2. Most common in arid and semi-arid regions with
   rapid erosion.
3. Sediment is coarse, poorly- sorted gravel and
   sand.

18
Nonmarine Environments

• Mudcracks
• Polygonal shape
• Form from alternate wetting and drying
• Associated with evaporites
• Halite
• Gypsum
• Anhydrite

19
Nonmarine Environments

• Waters move from mountains to sea through a
  variety of depositional environments
• Braided streams
• Meandering rivers
• Marginal marine systems

20
Fluvial Environments

• Braided and meandering river and stream systems
• River channels, bars, levees, and floodplains are
  subenvironments
• Channel deposits are coarse, rounded gravel, and
  sand.
• Bars are sand or gravel.
• Levees are fine sand or silt.
• Floodplains are covered by silt and clay.

21
River FeaturesMain Channel fed by tributaries
22
River FeaturesErosion dependent on rock type
23
SEDIMENT TRANSPORT BY STREAMS Because of the
role of rivers in erosion of the continents, they
must also carry this material downstream and
finally to the oceans, in order to remove it from
the continents. Rivers transport erosion products
in three ways A) In Suspension (clay, silt
kept in suspension by turbulence) B) In
Bedload (Gravel, sand, and silt) C) In
Solution (the components that enter solution
during erosion, dissolved salts)
24
River FeaturesTransport of particles via Gravity
25
Stream ErosionAndDeposition
26
Stream ErosionandDepositionUpland region
27
Stream Erosion and DepositionAlluvial Fans
28
Stream Erosion and DepositionBraided
Streamsabundant supply of coarse sediment
29
Stream Erosion and DepositionBraided
Streamsabundant supply of coarse sediment
30
Nonmarine Environments

• Braided streams
• More sediment available than the water can
  transport
• Forms numerous channels and bars

31
Nonmarine Environments

• Point bar
• Slowest flow on inner bend
• Accumulate sands
• Fastest flow on the outer bank
• Cuts away bank
• Natural levees
• Form during floods
• Coarsest sands deposited first, then fines

32
Rivers
33
Stream Erosion and DepositionMeandering
StreamsPoint Bar
34
Stream Erosion and DepositionMeandering
StreamsPoint Bars
35
Stream Erosion and DepositionMeandering
StreamsPoint Bars
36
Stream Erosion and DepositionMeandering
StreamsFlood Plain
37
Stream Erosion and DepositionMeandering
StreamsFlood Plain
38
Stream Erosion and DepositionMeandering
StreamsFlood Plain
39
Nonmarine Environments

• Meandering River
• Abundant water relative to sediment
• Backswamps
• Flood plain
• Mud settles out when stream overflows

40
Nonmarine Environments

• Vertical sequence of accumulation
• Coarse channel sediments at base
• Fine backswamp muds at top
• Illustrates Walthers Law

41
Transitional Depositional Environments

• Environments at or near the transition between
  the land and the sea.
• Deltas
• Beaches and barrier Islands
• Lagoons
• Tidal flats
• Estuaries

42
Depositional Environments
43
Deltas

1. Fan-shaped accumulations of sediment
2. Formed where a river flows into a standing body
   of water, such as a lake or the sea
3. Coarser sediment (sand) tends to be deposited
   near the mouth of the river finer sediment is
   carried seaward and deposited in deeper water.
4. The delta builds seaward (or progrades) as
   sediment is deposited at the river mouth.

44
Marginal Marine

• Delta
• Depositional body of sand, silt, and clay formed
  when river empties into the sea
• Sediments settle out in sequence

45
Marginal Marine

• Delta plain
• Layers of sand and silt deposited as river nears
  sea
• Distributary channels
• Separated by levees
• Delta front
• Silt and clay slope deposits
• Prodelta
• Clays often deposited by a freshwater plume
• Progrades into basin
• Sediments coarsen upward

46
Marginal Marine

• Mississippi River Delta
• River dominated delta
• Progrades into Gulf of Mexico
• Lobes
• Growing portion of the delta

47
Marginal Marine

• Mississippi River Delta
• Active lobe
• Growing portion of delta
• Switched lobes in the past
• Abandoned lobe
• Sediments compact
• Lobe sinks
• New lobe forms on top

48
Marginal Marine

• Deltaic Cycles
• Sequence of deposition
• Coarsens upward
• Erosion can remove tops

49
Deltas

• Mississippi River delta Niger River delta

50
(No Transcript)
51
(No Transcript)
52
Marginal Marine

• Barrier-Island Lagoon Complex
• Barrier Islands
• Waves and currents pile up sands
• Longshore Current
• Lagoons
• Protected from strong waves behind barriers
• Muds and muddy sands

53
Marginal Marine

• Progradation
• Shoreline builds out into sea
• High supply of sediment
• Builds over deeper water environments
• Illustrates Walthers Law

54
Marine

• Tempestites
• Storm deposits on shelf
• Sands deposited within normal muds and muddy sands

55
Marginal Marine

• Fossils
• Useful in reconstructing environments of past

56
Depositional Environments
57
Beaches and Barrier Islands

1. Shoreline deposits
2. Exposed to wave energy
3. Dominated by sand

58
(No Transcript)
59
Beaches and Barrier Islands

1. Marine fauna
2. A few km or less in width but may be more than
   100 km long
3. Separated from the mainland by a lagoon (or salt
   marsh)
4. May be associated with tidal flat deposits

60
Depositional Environments
61
Lagoons

• Bodies of water on the landward side of barrier
  islands
• Protected from the pounding of the ocean waves by
  barrier islands
• Contain finer sediment than the beaches (usually
  silt and clay)
• Lagoons are also present behind reefs, or in the
  center of atolls.

62
Sydney Australia
Palimco Sound NC
63
Tidal flats

1. Nearly flat, low relief areas that border
   lagoons, shorelines, and estuaries
2. Periodically flooded and exposed by tides
   (usually twice each day)
3. May be cut by meandering tidal channels
4. May be marshy, muddy, sandy or mixed sediment
   types (terrigenous or carbonate)

64
Tidal flats contd

• Laminations and ripples are common
• Sediments are intensely burrowed
• Stromatolites may be present (if conditions are
  appropriate)

65
Estuaries

• Mouth of a river drowned by the sea
• Brackish water (mixture of fresh and salt)
• May trap large volumes of sediment
• Sand, silt, and clay may be deposited depending
  on energy level
• Many estuaries formed due to sea level rise as
  glaciers melted at end of last Ice Age
• Some formed due to tectonic subsidence, allowing
  sea water to migrate upstream

66
(No Transcript)
67
Marine Depositional Environments

1. Continental shelf
2. Continental slope
3. Continental rise
4. Abyssal plain

68
Marine Depositional Environments
69
Marine Depositional Environments
70
Continental Shelf

• The flooded edge of the continent. Flooding
  occurred when the glaciers melted about 10,000
  years ago.
• Relatively flat (slope lt 0.1o)
• Shallow water (less than 200 m deep)
• May be up to 300 km wide (averages 80 km wide)
• Exposed to waves, tides, and currents

71
Continental Shelf contd

1. Covered by sand, silt, and clay
2. Larger sedimentary grains are deposited closer to
   shore.
3. Locally cut by submarine canyons (eroded by
   rivers during Ice Age low sea level stand)
4. Coral reefs and carbonate sediments may
   accumulate in tropical areas

72
Continental Slope

• The steeper slope at edge of the continent.
• Located seaward of the continental shelf
• Boundary between continental and oceanic crust
• May be about 20 km wide

73
Continental Slope contd

1. Deeper water
2. More steeply inclined (3 - 6o)
3. Rapid sediment transport down the slope by dense,
   muddy turbidity currents
4. Passes seaward into the continental rise

74
Continental Rise

• At the base of the continental slope.
• More gradual slope
• May be hundreds of km wide
• Water depths of 1400 to 3200 m
• Submarine fans form off submarine canyons
• Turbidity currents transport sediment downslope
  from continental shelf (turbidites)
• Passes seaward into the abyssal plain

75
Abyssal Plain

• The deep ocean floor.
• Nearly flat
• Water depths of 3 to 5 km (2 - 3 miles )
• Covered by very fine-grained sediment and shells
  of microscopic organisms
• Clay
• Volcanic ash
• Foraminifera (calcareous)
• Radiolarians (siliceous)
• Diatoms (siliceous)

76
(No Transcript)
77
(No Transcript)
78
(No Transcript)
79
Depositional Environments
80
Color of Sedimentary Rocks

• Black and dark gray coloration in sedimentary
  rocks generally indicates the presence of organic
  carbon and/or iron.
• Organic carbon in sedimentary requires anoxic
  environmental conditions.

81
Color of Sedimentary Rocks

• Red coloration in sedimentary rocks indicates the
  presence of iron oxides.
• Red beds typically indicate deposition in
  well-oxygenated continental sedimentary
  environments. May also be transitional or
  marine.

82
Color of Sedimentary Rocks

• Green and gray coloration in sedimentary rocks
  indicates the presence of iron, but in a reduced
  (rather than an oxidized) state.
• Ferrous iron (Fe2) generally occurs in
  oxygen-deficient environments.

83
Textural Interpretation of Clastic Sedimentary
Rocks

• Texture size, shape, sorting, and arrangement
  of grains in a sedimentary rock.
• The texture of a sedimentary rock can provide
  clues to the depositional environment.
• Fine-grained textures typically indicate
  deposition in quiet water.
• In general, it takes higher energy to transport
  larger grains.

84
Three "textural components" to most clastic
sedimentary rocks

• Clasts - the larger grains in the rock (gravel,
  sand, silt)
• Matrix - the fine-grained material surrounding
  clasts (often clay)
• Cement - the "glue" that holds the rocks together
  
• Silica (quartz, SiO2)
• Calcite (CaCO3)
• Iron oxide
• Other minerals

85
Grain Size

• Sedimentary grains are categorized according to
  size using the Wentworth Scale.

Gravel gt 2 mm
Sand 1/16 - 2 mm
Silt 1/256 - 1/16 mm
Clay lt 1/256 mm
86
Sorting

• Sorting refers to the distribution of grain sizes
  in a rock.
• The range of grain sizes in a sedimentary rock
  can provide clues to help interpret the
  depositional environment.
• For example, turbulence from waves will winnow
  out finer grain sizes such as silt and clay,
  leaving sands on the beach.

87
Sorting

• If all of the grains are the same size, the rock
  is "well sorted".
• If there is a mixture of grain sizes, such as
  sand and clay, or gravel and sand, the rock is
  "poorly sorted".

88
Sorting

• Well-sorted sands tend to have higher porosity
  and permeability than poorly-sorted sands (if
  they are not tightly cemented), and may be good
  reservoirs for petroleum and natural gas. Or good
  aquifers.

89
Sorting

• Poor sorting is the result of rapid deposition of
  sediment without sorting by currents. Examples of
  poorly-sorted sediment include alluvial fan
  deposits and glacial till.

90
Grain Shape

• Grain shape is described in terms of rounding of
  grain edges and sphericity (equal dimensions, or
  how close it is to a sphere).
• Shape of clasts is important in naming the
  coarser-grained sedimentary rocks.

91
Grain Shape

• Conglomerates have rounded clasts.
• If the particles are angular, the rock is a
  breccia

92
Rounding

• Rounding results from abrasion against other
  particles and grain impact during transport.
• Very well rounded sand grains suggest that a sand
  may have been recycled from older sandstones.

93
Sedimentary Structures

• Larger features which form during (or shortly
  after) deposition of the sediment, but before
  lithification.

94
Sedimentary Structures

• Some sedimentary structures are created by the
  water or wind which moves the sediment. Other
  sedimentary structures form after deposition -
  such as footprints, worm trails, or mudcracks.

95
Sedimentary Structures

• Sedimentary structures can provide information
  about the environmental conditions under which
  the sediment was deposited.
• Some structures form in quiet water under low
  energy conditions, whereas others form in moving
  water or high energy conditions.

96
Sedimentary Structures

• Stratification ( layering or bedding) is the
  most obvious feature of sedimentary rocks. The
  layers (or beds or strata) are visible because of
  differences in the color, texture, or composition
  of adjacent beds.

97
Graded Bedding

• The grain size in a graded bed is coarser at
  the bottom and finer at the top.
• Graded bedding results when a sediment-laden
  current (such as a turbidity current) begins to
  slow down.

98
Cross-bedding or cross-stratification

• An arrangement of beds or laminations in which
  one set of layers is inclined relative to the
  others.

99
Ripple marks

• Undulations of the sediment surface produced as
  wind or water moves across sand.
• Symmetric ripple marks are produced by waves

100
Ripple marks

• Asymmetric ripples form in unidirectional
  currents (such as in streams or rivers).

101
Mud cracks

• A polygonal pattern of cracks produced on the
  surface of mud as it dries.

102
Scour marks

• Depressions or erosional features formed as a
  current flows across a bed of sand.

103
Determining "up direction"
Rocks can be overturned by tectonic forces.
Examine sedimentary structures to determine "up
direction".

• Symmetrical ripples
• Stromatolites
• Burrows
• Tracks

• Graded beds
• Cross beds
• Mudcracks
• Scour marks

104
Sands and Sandstones

• Sandstone classification is based on the
  composition of the grains.

• Quartz
• Feldspar
• Rock fragments

105
Major types of sandstone

• Quartz sandstone - dominated by quartz
• Arkose - 25 or more feldspar
• Graywacke about 30 dark fine-grained matrix
• Lithic sandstone - quartz, muscovite, chert, and
  rock fragments. Less than 15 matrix.

106
Sandstone Interpretation

• Minerals provide information on the amount of
  weathering and transport of sand grains.
• Intense weathering and long transport produce
  sandstone dominated by quartz.
• Sandstones with abundant feldspars, and
  ferromagnesian minerals indicate relatively
  little weathering and transport.

107
Sandstone Environmental Interpretation

• Quartz sandstone
• Long time in the depositional basin
• Tectonically stable setting
• Shallow-water environments

108
Sandstone Environmental Interpretation

• Arkose
• Short time in the depositional basin
• Rapid erosion
• Arid climate
• Tectonic activity

109
Sandstone Environmental Interpretation

• Graywacke
• Tectonically active source area basin
• Rapid erosion

110
Sandstone Environmental Interpretation

• Lithic sandstone
• Deltaic coastal plains
• Nearshore marine environments
• Swamps or marshes

111
Deep Sea Environments

• Turbidity current
• Dense sediment-laden flow driven by gravity
• Turbidite
• Produces graded deposit
• Poorly sorted coarse grains at base
• Fine grains at top

112
Deep Sea Environments

• Turbidites are common in canyons
• Drop sediment load at base
• Form deposit similar to alluvial fan

113
Deep Sea Environments

• Pelagic sediment
• Fine-grained sediments that accumulate by
  settling through the water column
• Calcium carbonate
• Silica
• Clay

114
Deep Sea Environments

• Calcareous ooze
• Accumulations of single-celled planktonic
  organisms
• Foraminifera
• Calcareous nannoplankton
• Common lt 4 km w.d.
• Dissolution increases below 4 km w.d.

115
Deep Sea Environments

• Siliceous ooze
• Diatoms
• Radiolarians
• Common in upwelling regions
• Accumulations can alter to opal then chert

116
(No Transcript)
117
(No Transcript)
118
Carbonate Systems

• Organic reefs
• Modern reefs formed from coral
• Ancient reefs formed from different organisms

119
Carbonate Systems

• Reef front
• Seaward side
• Often rubble called talus

• Reef flat
• Lagoon
• On leeward side
• Patch reef

120
Carbonate Systems
121
Carbonate Systems

• Barrier reefs
• Elongate reefs with lagoon behind
• Fringing Reefs
• Grow along coast
• Lack lagoon

122
Carbonate Systems

• Atolls
• Reefs on volcanic islands
• Darwin
• Formed by sinking island
• Up to 65 km across
• Often open at one end

123
Carbonate Systems

• Buried atolls
• Often important petroleum reservoirs

124
Carbonate Systems

• Carbonate Platform
• Broad carbonate structure above seafloor
• Windward side
• Nutrient rich
• Abundant reefs
• Buffered system
• CO2 H2O H2CO3
• HCO3 CaCO3 2HCO-3

125
Carbonate Systems

• Stromatolites
• Cyanobacteria mats trap sediments
• Grows up through sediments to produce new one
• Layered organic-rich and organic-poor muds

126
Carbonate Systems

• Living Stromatolites
• Found in hypersaline, supratidal and intertidal
  settings
• Little competition and predation
• Shark Bay, Australia